Electrical heating elements



P 0, 1966 w. G. MATHESON ETAL 3,274,374

ELECTRICAL HEATING ELEMENTS 2 Sheets-Sheet 1 Filed May 7, 1965 JAMES PCLUNE THEODORE J. PRICENSKI N O S E H T A M G m R F W INVENTORS FIG.3

ATT RNEY p 1966 w. G. MATHESON ETAL 3,274,374

ELECTRICAL HEATING ELEMENTS Filed May '7, 1963 2 Sheets-Sheet 2.

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WILFRID G. MATHESON JAMES F. CLUNE THEODORE J. PRICENSKI l VENTORSUnited States Patent 3,274,374 ELECTRICAL HEATING ELEMENTS Wilfrid G.Matheson, Marblehead, James P. Clune, Danvers, and Theodore J.Priceuski, Ipswich, Mass, as-

signors to Sylvania Electric Products Inc., a corporation of DelawareFiled May 7, 1963, Ser. No. 278,660 11 Claims. (Cl. 219-426) Thisinvention relates to refractory metal heating elements for electricalfurnaces and more particularly to heating elements which are suitablefor use as anodes or susceptors. Such elements are particularly usefulin high temperature electric discharge furnaces or induction heatingfurnaces respectively.

Anodes or susceptors are well known to the art as heating elements andcertain of them have been previously fabricated of refractory metals. Inthe past, however, elements using refractory metals have been fabricatedfrom sheet materials or machined from heavy stock and when so fabricatedthey do not possess crystal structures having optimum characteristics toresist breakage or maintain their geometric form. But we have discoveredthat refractory metal wire does possess these characteristics and whenproperly fabricated, the elements may be used successfully as anodes forhigh temperature discharge furnaces or susceptors for induction heatingfurnaces utilizing a vacuum and/or inert or reducing atmospheres. Thisdiscovery has led to the achievement of long-lived, rugged heatingelements which can withstand temperatures as high as 2.000" C. and oftenas high as 2500 to 3000 C. When using refractory metal wire, theelements can have substantially identical electrical characteristics asthose known in the .prior art, but yet be structurally far superior totheir solid sheet counterparts.

High temperature electric discharge furnaces operate by forming athermionic electron emission from a cathode which surrounds the anode ofthis invention. When adequate temperature is reached, the ions insteadof being supplied from the heated surface can be supplied from acontrolled flow of gas into the discharge region between the anode andthe cathode or from direct emission in a vacuum. In gas containingfurnaces, the gas is ionized by collision with the electrons and bycontrolling the number of ions so released the temperature may beregulated. The positive ions liberated tend to neutralize the spacecharge between the cathode and the bombardment surface. They also act ascurrent carriers, as do the released electrons, increasing the current,but the carriers added relatively few; by far the greater proportion ofthe current is carried by initial thermionic discharge. The latter caseis substantially exclusively true in high vacuum furnaces. The dischargeis diffuse and tends to spread over the entire surface of thebombardment anode, and since it is diffuse there is no concentration ofthe current on a single spot to form an arc crater, whereby the anodewould be vaporized to form a true arc. Heating of the cathode ispreferably initiated by passing current through it directly, although itmay be indirectly heated when desired. Heating of the cathode alsooccurs due to positive ion bombardment and by radiation from the anodeand hence after the discharge has been maintained for a long enoughperiod for equilibrium temperatures to establish themselves, the cathodetemperature is maintained partly by heat from these sources and thepower supplied to it directly can be reduced.

Induction heating has been extensively applied in industry for melting,heat treating,brazing and soldering, hot forming and other processingoperations. In such applications, the susceptor which is electricallyconductive, is generally heated directly by induced current when3,274,374 Patented Sept. 20, 1966 it is placed in an electromagneticfield established by a high frequency current flowing through asurrounding induction heating coil. The article of work is placed withinthe susceptor and heated by radiation and in recent years the use ofsuch susceptors has significantly extended the application of inductionheating. In its simplest form, a susceptor can be a metal tube havingmagnetic characteristics interposed between an electromagnetic coil andthe article of work to be heated. The susceptor is then heated by themagnetic field established by the induction coil but the formeressentially shields the work within it so that the element being treatedcan be heated primarily by radiation and/ or conduction from the heatedsusceptor. Not only can electrically conductive elements be heatedthusly, but also many non-conductive materials such as ceramics andplastics.

The susceptors or anodes according to our invention can take a number ofdifferent forms and shapes. In the preferred embodiment, a number ofhelically convoluted wires (helical coils) each having generallysubstantially similar diameters are intertwisted together in such a waythat generally two convolutions are intertwisted in each otherconvolution. In this manner we continue the intertwisting until aforaminous plexus is formed. Preferably, the plexus is then bent aboutthe axes of the helical coils into a cylinder and welded at its abuttingends to form a seam. The diameter of the cylinder can vary withoutlimit, depending only upon the size of the furnace in which it is to beused. Large diameter wires can be used for larger cylinders and smallerdiameter wires for smaller cylinders if desired.

The use of a series of intertwisted helical convolutions allows greaterlatitude in the application of the anode or susceptor. Even afterheating and thus when each of the individual convolutions is quitebrittle, they are each movable in their adjacent convolutions and hencecan withstand stresses which would ordinarily fracture solid sheets ofsimilar refractory metals. Coupled with the inherent flexibility ofplexuses is the increased strength resulting from drawn wire over solidsheets due to the incorporation of long fiber-like crystals as a resultof processing. Thus even though the individual convolutions of wire maybecome quite brittle after the element has been used once, the plexusitself it still fairly flexible and can be moved in the furnace withoutan inordinate danger of breaking. Furthermore, the problem of thermalshock, usually resulting from heating too rapidly or cooling too quicklyis materially reduced due to the construction and crystal structureprovided by wire. When desired for greater stability, and whenflexibility can be dispensed with as a criteria, our plexuses may serveas a base for a thicker coating of flame sprayed refractory metals whichmay be prepared according to conventional techniques. The use of flamesprayed coatings is not generally practical with solid sheet anodes orsusceptors because flame sprayed material will not permanently adheretosuch bases.

Accordingly, the primary object of this invention is the fabrication ofheating elements having increased meohanical strength and the ability towithstand fairly large thermal shocks without breakage.

Another object of this invention is the substantial elimination ofdistortion in the geometrical shape of anodes and susceptors after theirelevation to high temperatures in furnaces.

Another object of this invention is the extension of the life ofelectrical heating elements fabricated from refractory metals which canbe heated to temperatures as high as about 3000- C.

A feature of this invention is the fabrication of a susceptor or anodefrom a plexus of refractory metals, the

plexuses comprising a series of intertwisted helical convolutions of themetal.

An advantage of this invention is that the anode or susceptor formed ofa foraminous plexus can have a longer life and be stronger after heatingthan similar elements fabricated of sheet metal or machined from heavystock.

Many other objects, features and advantages of this invention willbecome manifest to those conversant with the art upon making referenceto the detailed description which follows and the accompanying sheets ofdrawings in which preferred embodiments of susceptors or anodes of arefractory metal, foraminous plexus are shown and described and whereinthe principles of the present invention are incorporated by way ofillustrative examples. Of these drawings:

FIGURE 1 is a perspective view of the preferred embodiment of the anodeor susceptor.

FIGURE '2 is an enlarged fragmentary view of the helical intertwistedwires which form one embodiment of the anode or susceptor shown inFIGURE 1. These wires are joined together at their ends by a weld whichis schematically illustrated.

FIGURE 3 is an enlarged fragmentary view of the helical intertwistedwires which form another embodiment of the anode or susceptor shown inFIGURE 1.

FIGURE 4 is a cross-sectional view of our susceptor disposed within anelectromagnetic-type furnace.

FIGURE 5 is a cross-sectional view of our anode disposed within a hightemperature electric discharge furnace.

FIGURE 6 is a cross section of an anode or susceptor which has beenmetallized (51) with a spray coating on the plexus (52).

In the preferred embodiment shown in FIG. 1, the anode o-r susceptorelement according to our invention can easily be fabricated by forming aplexus 1 of a series of inter-twisted, individual wire helixes. Thehelixes may 'be formed of any of the usual refractory metals such asmolybdenum, columbium, taut-alum, rhenium or preferably tungsten.Additionally, alloys of such metals having requisite melting points alsohave applicability in some cases. The wire diameter ordinarily should beabout 0.010 to 0.125" since such wire sizes ofrer optimumcharacteristics in a furnace. Bel-ow about 0.10", the element fabricatedof these metals will volatilize too readily when heated due to the largesurface area and hence, the life of the element will be drasticallyreduced. Above about 0.125" the wire will be difficult to work and coil.The thickness of the plexus will vary depending upon the internaldiameter of the helix together with the diameter of the wire. It isdesirable for most applications of the elements to form the helicalconvolutions on m-andrels having diameters of about 0.025 to 0.500.While the upper limit may be increased to suit individual furnace designrequirements, it is generally not feasible to go below the lower limitsstated because the wire which will have to be used will be too fine tomake an efficient element having reasonable life. Pitch of theindividual convolutions can vary from about 300% (that is, the spacingbetween the turns equaling slightly more than two times the diameter ofthe wire) to about 1000% or even greater. It is apparent however that atthe upper limit, the wires must have suflicient pitch to allow forintertwisting of several of the convolutions together. Preferably formost applications, we use a pitch of about 300% so that a tight plexusis formed. Now we have stated that fine wire should not be used becauseit would reduce the life of the element, however it must be pointed outthat when the plexus is flame sprayed with the suitable material, evenfiner wires may be used since internal strength in such cases is notsuch an important prerequisite.

Intertwisting of the wire helixes may be performed easily by disposingone wire in a jig and then rotating a second wire helix into the turnsof the first. A third wire helix is then intertwisted in the turns ofthe second helix and the operation is repeated again and again until aplexus of the desired width and length is attained. Although we show aconfiguration having two convolutions intertwisted in each otherconvolution, many other variations can be used also. For example, thesingle wire shown can :be doubled and possibly tripled and themulti-stranded wire wrapped into helical convolutions of the desiredpitch on a mandrel. Other modifications include insertion of stranded orstraight wire into the turn abutments of the convolutions to affordadditional strength. And yet, another method of intertwisting wireinvolves placing the convolutions of one coil into the interdigitalspaces between the convolutions of another coil and then joining theconvolutions together by threading a straight wire through the twoconvolutions, which process can be repeated until a plexus of a desiredsize is obtained.

Because in the prepared embodiment, the plexus will have to be shapedafter it is formed, it is generally desirable to use coils which havenot been stress-relieved. Such coils have not yet been treated to setthe crystal structure and hence may be bent and shaped as desiredwhereas stress-relieved coils are brittle. After the finalconfigurations of the element are made, the entire unit may then beheated to set the crystal structure.

When a plexus of requisite size is fabricated, it is then ready to beformed into the desired shape. Preferably, the coils of the plexus arebent into a generally cylindrical shape on a plane substantiallyperpendicular to their axes. When the desired shape is obtained, and ifa regular, continuous shape is desired, a weld is formed by conventionalheliarc methods at the edges of the plexus and on the distal ends of theindividual wire helixes.

In the applications of the anodes or susceptors, a generally cylindricalshape is most desirable. Such shapes are preferably cylindrical althoughrectangular, polygonal or irregular shapes may be fabricated easily. Thelatter rectangular, polygonal or irregular shapes have particularapplication where the article which is to be heated has similarrectangular, polygonal or irregular shapes. As an example, the advantageof heating a rectangular article of work with a rectangular element isthat uniform spacing between the article and the element is obtained.While a cylindrical shape might be practical for heating most articlesor work which are small in comparison to the diameter of the element orfor heating large cylindrical objects, the relationship is not alwayspractical when heating large pieces of work of shapes different than theelement. In these cases the other noncylindrical shapes should be used.

For some particular applications it may be desirable to heat one sectionof work to a higher temperature than the other adjacent sections. Suchtemperature differentials may be easily attained by shaping the anode 0rsusceptor so that a section thereof would be closer to the article orwork than the remainder of the element. Due to greater proximity to thework, greater heating will be attained at the point which is nearest tothe element than at adjacent points which are more distant.

Referring now to the detailed view of FIGURE 2, it will be seen that thecoils, 3, 4, 5, are intertwisted in such a manner that the upper coil 3is intertwisted in the intermediate coil 4 which in turn is intertwistedin the lower coil 5. In the embodiment shown, the turns of the coils arespaced fairly closely together (300% pitch) in order to form a denseforaminous plexus.

In order to join the ends of the cylinder together and form a seam, wehave previously indicated that a heliarc weld also may be used, howeverother means of securing the ends together such as mechanical clampingmay also have applicability so long as such means can withstand thetemperatures to which the element will be raised without appreciablewarping, distortion or alteration of the electrical characteristics.

While a seam is the preferred method of forming the plexus into agenerally cylindrical shape, we may also obtain this shape byintertwisting a helical convolution of refractory metal wire similar tothe adjacent helical convolutions 8 and 9 into the first and last coils6 and 7 of the plexus as shown in FIGURE 3.

Sometimes it is desirable to use a basket-type element particularlywhere there are many small articles of work to be heated together. Theseelements may be easily formed by placing a bottom on the cylindricalelement either by heliarc welding a flat, round plexus to the endthereof or using other suitable means of attachment. Of course, evensheet tungsten may be used, however sheets on the bottom suffer from thedifiiculties previously indicated. They will tend to warp after heatingand become brittle.

Frequently when ruggedness is a critical factor, such as would be thecase if the element is to be raised to elevated temperatures quitefrequently and possibly for short durations of time, it may be desirableto heliarc welda reinforcing rim (not shown) around the edges. The rimmay be simply a tungsten flux if tungsten is used as the elementmaterial, or possibly, it may be fabricated of a ring of suitablerefractory metal.

As we have indicated previously, the plexus 52 forming the anode orsusceptor can be metalized by spraying a molten metal onto the surfaceto form a coating 51 as shown in FIGURE 6. When using a tungstenelement, it is generally preferred to coat tungsten thereupon. Otherrefractory metal plexuses can use similar refractory metal coatings. Theprocess involves melting the metal in a flame and atomizing it by ablastof compressed air into a fine spray. This spray builds up onto thesurface to form a solid metal coating. Because the molten metal isaccompanied by a large blast of air, the object being sprayed is notheated sharply. When the element of our invention is metalized, theflexibility is lost, however strength is markedly increased. Even when afurnace temperature of 3000" C. is used, there is such a quantity oftungsten present that the life will :be very long. Furthermore just asreinforced concrete has a greater strength is markedly increased. Evenwhen a furnace is a metalized heating element formed upon a foraminousplexus.

Referring now to FIGURE 4 of the drawing, a typical induction furnace isshown wherein the susceptor of our invention is illustrated. Theprincipal equipment of an induction heater can be a coil or wire throughwhich an alternating current flows. The alternating current induces eddycurrents in any metal that is either located within the coil or closelysurrounds the coil. The frequency ranges between 60 c.p.s. and 3megacycles and even higher in certain special applications. Heat isgenerated in the metallic article or work by induction regardless of thecomposition of any non-metallic materials which are disposed between thearticle of work and the coil. The coils are generally Wound fromflattened copper pipes through which water is circulated.

In the furnace, a flow of inert and/or reducing gas can be passedthrough the inlet port 10' over the members within the furnace which areto be heated and removed from an outlet port '12. In cases where avacuum is to be maintained, the reduction of pressure can be attained bywithdrawing gases through the outlet port 12 while the inlet port 10 isclosed. The furnace 15 is lined with insulation 14 to reduce heat:losses and when desired, a series of heat shields (not shown) can bedisposed immediately inside of the insulation 14.

An internally water cooled helical coil 16 is attached to the source ofcurrent (not shown) by conventional techniques and the susceptor 18according to our invention is disposed inside of the helical coils 16.As shown, the susceptor can have a generally cylindrical shape and canbe fabricated according to the description indicated previously.Suitable supports (not shown) are provided within the furnace 15 so asto hold the susceptor 18 axially aligned within the helical, watercooled coil 16. An article of work (not shown) can be supported,suspended or disposed within the susceptor 1-8, as is conventional inthe art.

While the preferred embodiment of our invention involves using theforaminous plexus which is bent on a plane perpendicular to the axes ofthe individual helical convolutions which are used to make it, it isapparent that other modifications may be used equally well. Frequentlyit is possible to use convolutions which need not be shaped until afterthe fabrication of the plexus. Such arrangement can easily be made bybending the plexus in a plane substantially parallel to the individualconvolutions and then joining the ends together with a similar wire coilwhich will act as a retainer. In this manner an entirely cylindricalelement can be formed without the necessity of welding.

The embodiment of our invention in 'which the heating element is used asan anode is shown in FIG. 5. In this embodiment, a furnace 20 whichincludes a layer of insulation 21 is used to surround the entire heatingunit so as to contain the heat therein. Supported upon three buss bars,22 and 23 (the third buss bar is not shown in this cross section) is acathode which may be formed according to our co-pending application Ser.No. 219,404, filed August 27, 1962 entitled Electrical Heating Elementand assigned to the same assignee as the instant application, now UnitedStates Patent 3,178,665. As therein described the heating element cancomprise three grates 24, 25, and 38. The anode is preferably assembledin a substantially cylindrical form and can be connected together at oneend by an outer conductor ring 27 and an inner conductor ring 28.Preferably the attachment of the grates to the conductor rings is madeby heliarc welding the rings together on their lowermost extremity.

At the other end of the cathode are supporting, electrically conductingarms 29 and 30 which extend radially upwardly from the heating element.These supporting arms 29 and 30 are adapted to be disposed withinelectrically conducting buss bars 22 and 2 3. Since 'we prefer to makethe supporting arms of at least two portions and preferably four asdescribed in the above-mentioned application, we have found that it isfrequently desirable to tie the portions together with windings 32 and33. Such windings are preferable since they afford radiant heatdissipation at the outward extremities of the arms and help to preventoverheating of the buss bars. Disposed inside of the grates 24 and 2-5,at the upper end thereof, are inner support segments 34 and 35 which arejoined to the grates 24 and 25 and in turn to the support arms 29 and 30by a flux-type weld 36 and 37. The grate 38 and the rest of itsstructure is similar to that of either grates 24 and 25.

Surrounding the cathode are a series of heat shields 40 which may varyin number depending upon the heat to which the furnace is to be raised.These heat shields 40 are generally supported upon a rod or othersuitable support device 42 which may be aifixed to the refractory liner43. A water cooling system 44 can be advantageously used to keep thetemperature of the furnace within reasonable limits and to preventlocalized overheating.

The anode 4-5 of our invention can be conveniently disposed within thecathode in such a manner that the axis thereof substantially coincideswith the axes of the anode. Any articles of work which are to be heatedcan be supported, suspended or disposed within the anode 45 in a mannerwhich is conventional in the art. The anode itself can be convenientlysupported upon a stand 46 which is in turn supported upon refractorystruts 47. The anode is placed in the electrical circuit by passingcurrent to the stand 46 through means well known in the art. Disposedbeneath the anode are a series of heat shields 48 which radiate heatupwardly into the furnace.

While there is shown and described herein certain specific structureembodying our invention, it will be manifest to those skilled in the artthat various modifications and rearrangements of the parts can be madewithout departing from the spirit and scope of the underlying conceptand that the same is not to be limited to the particular forms of theinvention herein shown and described, except insofar as indicated by thescope of the appended claims.

As our invention we claim:

1. A heating element comprising a generally cylindrical plexus formed ofa multiplicity of intertwisted helical convolutions of refractory metalwires, said cylindrical shape being formed by intertwisting a helicalconvolution of a refractory wire similar to the other helicalconvolutions in the plexus into the first and last convolutions thereof,

2. The heating element of claim 1 wherein the generally cylinderoida-lshape is formed by intertwisting a helical convolution of refractorymetal wire similar to the other helical convolutions in the plexus intothe first and last convolutions thereof.

3. The heating element according to claim 1 wherein the refractory metalis tungsten.

4. The heating element according to claim 1 wherein the refractory metalwires have a diameter of about 0.010 to 0.125 inch.

5. In a high temperature electric discharge furnace having a hollowcathode disposed therein, the improvement which comprises: an anodedisposed inside said cathode and spaced therefrom, said anode being agenerally cylindrical plexus formed of a multiplicity of intertwistedhelical convolutions of refractory metal wires.

6. A high temperature electric discharge furnace according to claim 5wherein the cylindrical shape of the anode is formed by joining togetherthe distal ends of the multiplicity of intertwisted helicalconvolutions.

7. The high temperature electric discharge furnace according to claim 5wherein the generally cylindrical shape of the anode is formed byintertwisting a helical convolution of refractory metal wire similar tothe other helical convolutions in the plexus into the first and lastconvolutions thereof.

8. The high temperature electric discharge furnace according to claim 5wherein the refractory metal wire has a diameter of about 0.010 to 0.125inch.

9. The high temperature electric discharge furnace according to claim 5wherein the anode has a flame sprayed coating of refractory metaldisposed upon the outer surface thereof.

10. The heating element according to claim 9 wherein the flame sprayedcoating and the refractory metal wires are formed of tungsten.

11. A heating element comprising a generally cylindrical plexus formedof a multiplicity of intertwisted helical convolutions of refractorymetal wires said cylindrical shape being formed by welding together thedistal ends of the multiplicity of intertwisted helical convolutions.

References Cited by the Examiner UNITED STATES PATENTS 69,193 9/1867Dreusike 5-189 X 223,262 1/1880 Wakernan 245-5 2,686,212 8/1954 Horn etal. 13-27 2,848,523 8/1958 Hanks et al. 13-31 2,908,739 10/1959 Rummel13-27 3,020,387 2/1962 Bosche et al 219- 3,056,881 10/1962 Schwarz219-50 3,172,002 3/1965 Johnson et al 313-348 X 3,178,665 4/1965*Matheson et al. 338-208 X RICHARD M. WOOD, Primary Examiner.

L. H. BENDER, C. L. ALBRITTON,

Assistant Examiners.

5. IN A HIGH TEMPERATURE ELECTRIC DISCHARGE FURNACE HAVING A HOLLOWCATHODE DISPOSED THEREIN, THE IMPROVEMENT WHICH COMPRISES: AN ANODEDISPOSED INSIDE SAID CATHODE AND SPACED THEREFROM, SAID ANODE BEING AGENERALLY CYLINDRICAL PLEXUS FORMED OF A MULTIPLICITY OF INTERTWISTEDHELICAL CONVOLUTIONS OF REFRACTORY METAL WIRES.